Book excerpt: Due to an increase in traffic levels over the last 20 years, the costs and disruption of roadworks have increased proportionately. This has led to a re-evaluation of previous standards, and a proposal that a design life of 40 years for heavily trafficked areas should become the new standard. This report reviews design practice and information on flexible pavement performance, and develops an improved design method for heavily trafficked, flexible pavement. design criteria and design concepts. A well constructed pavement, built above threshold strength, will have a very long structural service life provided that distress, in the form of cracks and ruts appearing at the surface, is treated before it begins to affect the structural integrity of the road.